1. Field of the Invention
The present invention relates to the preparation of ferric chloride and, more especially, to the preparation of ferric chloride by chlorination of aqueous solutions of ferrous chloride.
2. Description of the Prior Art
Ferric chloride is a known compound which is useful as a flocculating agent in water treatment. Compare, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd edition, volume 24, pages 394-396 (1984) and volume 10, page 498 (1980).
The simplest process for the preparation of ferric chloride entails digesting iron with concentrated hydrochlorine acid; an aqueous solution containing approximately 36% by weight of ferrous chloride (FeCl.sub.2) is thus obtained, and this is chlorinated to produce an aqueous solution of ferric chloride (FeC1.sub.3) which analyzes by titration at approximately 41% by weight. This 41% solution can be directly employed as a flocculating agent, and is the usual commercial form. A concentrated solution of FeCl.sub.2 must be produced, because FeCl.sub.2 and FeCl.sub.3 can undergo partial hydrolysis during concentration by evaporation, to give HCl. The presence of HCl in FeCl.sub.3 is a particular problem in water treatment. The above process also requires the use of concentrated hydrochloric acid.
U.S. Pat. No. 3,682,592 describes a process for producing ferric chloride in which the solution of ferrous chloride is contacted with oxygen.
U.S. Pat. No. 4,066,748 describes a process for preparing ferric chloride beginning with a solution of FeCl.sub.2 emanating from a descaling bath. This process requires both a concentration of the ferrous chloride as well as a two-step chlorination.
In Example 3, the '748 patent indicates the impossibility of chlorinating the ferrous chloride completely using the stoichiometric amount of chlorine when using a single reactor, or the requirement to employ an excess of chlorine to chlorinate all of the ferrous chloride.
In carrying out the reaction scheme detailed in Example 4 of the '748 patent, the chlorination of FeCl.sub.2 to FeCl.sub.3 is reported, but in two reactors, which are charged with a FeCl.sub.2 /FeCl.sub.3 mixture.